Surface hardening of titanium



United States Patent SURFACE HARDENING OF TITANIUM Andrew J. Griest and Warren M. Parris, Columbus, and Paul D. Frost, Groveport, Ohio, assignors, by mesne assignments, to the United States of America as represented by the Secretary of the Army No Drawing. Application January 29, 1953 Serial No. 334,086

Claims. (Cl. 148-131) This invention relates to surface hardening of titanium. It has to do, more particularly, with a gas carburizing method for producing hard surfaces on titanium and titanium-base alloys, and with hard-surfaced articles produced by such method.

A primary object of this invention is to provide a method for forming a hard, adherent surface layer of titanium carbide (TiC) on articles made of titanium or titanium-base alloys, in order to eliminate the tendency oftitanium or titanium-base alloys to gall or seize when used in moving contact with other metal surfaces, and to produce such surfaces with a high degree of wear resistance.

Methods now known to the public for forming hard, Wear-resistant layers on titanium include (1) treatment in molten cyanide baths, and (2) a process for forming a thin layer of tungsten carbide on titanium objects.

Thesurface hardening of titanium and titanium-base alloys by treatment in molten cyanide baths has been described by R. S. Dean in Us. Patent No. 2,453,896, (1948). The process consists of treating the objects to be surface-hardened in a molten bath of an alkali metal cyanide (presumably NaCN or KCN) at an elevated temperature. For NaCN, this temperature presumably lies between 1046 F. and 1580 F., the melting point and decomposition temperature respectively. The patent states that the hardening can be produced to any depth, and that the resultant case is ductile to a high degree and in the same hardness range as hardened steel and tungsten carbide.

A method of coating titanium and other metals with a layer of tungsten carbide has been reported. Journal of Metals, October 1952, pp. 1036-1037, New Process Permits Many Metals to'be Tungsten Carbide Coated (no author given). This process is reported to be capable of developing cases from 0.000S-inch to 0.02-inch in depth on titanium parts. The coating is said to have wear resistance equal to that of sintered tungsten carbide. An advantage claimed for the process is that the base inetal does not have to be heated above 400 F. during the process. The method is not described, but evidently involves the application of a complex cobalt-tungsten carbide powder to the surface of the part to be coated. The bond between the coating and the base metal is said to be mechanical in nature and does not involve fusion or welding at the interface.

The present invention provides improvements over the foregoing methods of surface hardening titanium in the following respects:

(1) The surfaces produced on titanium and titaniumbase alloys by the method of this invention are harder and more wear resistant than the cases produced by treatment in molten cyanide.

(2) The method of this invention does not involve the use of highly poisonous cyanide compounds.

(3) The method of the present invention is simpler and less expensive to carry out than the process of coating with tungsten carbide.

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It is another important object of this invention to provide surface-hardening methods having the foregoing advantages.

It is a further object to provide articles made substantially of titanium and provided with a hardened surface layer.

Other objects and advantages will be apparent from the disclosure herein.

A process of surface hardening for articles made of titanium and titanium-base alloys according to the present invention comprises subjecting such articles to an atmosphere containing a mixture of propane and argon at an elevated temperature, followed by cooling the articles at a controlled rate in a controlled atmosphere.

The type of case formed possesses a titanium carbidetype structure, while the maximum depth of case formed depends primarily on the treatment temperature. This type of case structure is different from those formed in the other described surface-hardening treatments in that the case consists essentially of TiC, while the treatment in molten cyanide baths forms a case which is primarily alpha titanium containing dissolved nitrogen, and the other treatment produces a coating of tungsten carbide.

The gas carburizing process of this invention provides a method for carrying out a chemical reaction between nascent or activated carbon and the surface of an essentially titanium article being treated, in accordance with the reaction Ti-j-CiTiC (AF=-54 k. caL/mol. at 1750 F.)

As diffusion of the carbon proceeds inward through the metal, a layer of TiC is formed near the surface of the metal.

Although the process as described above refers to the use of propane-argon mixtures as the treatment atmos phere, other suitable media may be used. The treatment atmosphere preferably comprises the following components:

A. The carburizing agent, which is a compound of carbon not containing oxygen or nitrogen. Suitable compounds of this class are hydrocarbons which preferably are gaseous at standard temperature and pressure. Examples, in addition to propane, are methane, ethane, butane, and higher members of the series C H which can be rendered in the gaseous form by suitable means. Other compounds that may be used are unsaturated hydrocarbons of the form C H Such as ethylene and propylene. The carburizing agent may comprise a single gas of the types mentioned above, or a mixture of any number of such gases.

B. An inert carrier gas which serves as a diluent of the carburizing agent. This may include any gaseous element or compound which does not react with titanium or titanium alloys. Suitable gases of this type are argon, helium, neon, and krypton.

The treatment variables having the greatest effect upon the extent and nature of the surface-hardening reaction are.

(1) Concentration of the active carburizing agent.

(2) Temperature.

(3) Time.

(4) Rate and mode of cooling.

(5) Prior surface finish of the object treated.

(6) Flow rate of gas mixture.

The following preferred treatment conditions using a propane-argon mixture have been established: Concentration-0.5% to 1.0% propane by volume, the

balance essentially argon.

Temperature1700 F. to 1800 F.

Time-6 to 10 hours.

Cooling-A rate approximating that of an air cool, carried out in an argon atmosphere.

Prior surface finish--A ground surface is most favorable.

A highly polished surface has been found to be undesirable.

Over-all flow rate of gas-This should beat least several times the calculated fiow'required to form the'desired depth ofcase inthe" appropriate time; In particular; specimens with a total surface area of about 20.0'square 1 centimeters can be carburized to a depth' of about 0.0002 inch' in 8 hours at 1750 F. using'a 08% propane-balance argon mixture, and using a flow rate" of about 2.2 cubic centimeters'per minute of propane, metered at 70 F. and 14.7 pounds'per' square inch absolute.

Preferably the concentration of the propane should be limited to less than about 1% by volume, sincesooting and graphite deposition. may be encountered on the work at higher concentrations; These deposits apparently form a layer which impedes reaction of nascent or activated carbon with the metal.

The optimum temperature of the carburizing gas is in the neighborhood of 1750 F. At temperatures of 1450 F. to 1600" F., only very shallow cases are formed after eight hours of treatment. At'higher temperatures, around 2000 F., much heavier cases are formed, but are badly pitted, have poor adherence, and have extremely rough and irregular surfaces.

At a given temperature, it appears that the depth of case approaches a maximum limit asymptotically with time. At 1750 F., this maximum is essentially reached after four to eight hours.

Cooling should be carried out in an inert atmosphere; Good results may be obtained with furnace cooling, but as this is time consuming,.a more rapidcool, such as that approximating the rate of an air cool is generally preferable, more rapid cooling or cooling in an active atmosphere may cause spalling and marring of the surface.

The carburizing reaction may be carried out in any suitable chamber made of a material which is capable of being heated to temperatures in the carburizing range and which does not react in a detrimental fashion with the carburizing atmosphere. The reaction chamber is provided with inlets and outlets for the carburizing gases, the flow of the gases being measured with appropriate instruments. The surfaces of the articles being treated should have substantially uniform and unobstructed access to the carburizing atmosphere.

The properties of the cases formed by carburizing titanium with propane-argon mixtures under preferred conditions are summarized below:

A.. Structure-TiC-type lattice, not containing the stoichiometric quantity of carbon (about 20%), but a lesser amount estimated to be from about 8% to about 12% carbon.

B. Hardness-Generally in the range of from approximately 1200 to'approximately 1500 Knoop (-100 g. load).

C- Optimum depth-About 0.0002 inch.

D. Adherence-Satisfactory. Surface-hardened specimens can be handled and set up ina wear-test apparatus without spalling.

E. Surface appearance-Dull, metallic gray.

F. Wear resistance-On the basis of laboratory-wear tests, the TiC-type case produced by carburizing with propane-argon was found to be superior in .wear resistance to the type of case produced by treatment in a molten cyanide bath.

G. Galling tendency-Less than that of untreated ti tanium on the basis of laboratory wear tests.

It is apparent from the foregoing disclosure that a novel method has been provided for forming a hard, adherent surface layer of titanium carbide on articles made of titanium or titanium-base alloys, the titaniumcarbide surface layer. having a high degree of wear re sistance andobviating galling orseizing when in'moving contact with another metal surface. This invention further provides articles made. substantially of titanium and having hardened, wear-resistant surfaces. It will be understood, of course, that, while the forms of the gas carburizing method and the articles thereby provided as herein described constitute preferredemb'odiments of the invention, it is not intended herein to illustrate all of the possible equivalent forms or ramifications of the inven tion.

the spirit or scope of the invention herein disclosed.

What is claimed is:

1'. A method of surface hardening for articles made of titanium and'titanium base alloys comprising the steps of subjecting such articles to an atmosphere comprising from about 0.5% to about 1.0% of at least' one hydrocarbon gas by volume of the series consisting of 'C,,H and C H and the balance essentially argon at a temperature in excess of about 145 0 F.

2. A method as defined in claim 1 in which saidtemperature is in the range of from about 1700" to about 1800 F.

3. A method of surface hardening articles made of titanium and titanium-base alloys comprising providing. a ground surface onv each such article, and subjecting said articles to. an atmosphere. comprising from about 0.5% to about 1.0% propane by volume and the balance essentially argon at a temperature in the range of from about 1700 F. to about 1800 F.

4. A method of surface hardening articles made of titanium and titanium-base alloys comprising subjecting such articles to an atmosphere comprising a mixture of approximately 0.8% propane and the balance essentially argon at a temperature of approximately 1750 F. for approximately 8 hours, with a flow rate of said propane mixture of' approximately 11 cubic centimeters per minute for each 100 square centimeters of surface area on said articles metered at approximately 70 F. and approximately'14'.7 pounds per square inch absolute.

5. An article made substantially of titanium. and provided'with a hardenedsurface bythe method described in claim 1.

References Cited in the file-of this patent UNITED STATES PATENTS 1,817,345 Brower- Aug. 4, 19.31 r 1,974,541 Ostennan Sept. 25, 1934 2,546,320 Rostrom Mar. 27, 1951 OTHER REFERENCES PB Report 108,892, March.3l, 1952. (Availableat Library of Congress.)

It will also be understood that the words usedarev words of description rather than of limitation, and that various changes may be made without departing from 

